An ignition circuit of the kind described above and equipped with a microprocessor as a control circuit was introduced in Sweden in a motor-driven chain saw known as Jonsered 2051. The microprocessor adjusts the ignition for every speed range in order to assure an optimal ignition. However, if there is an acceleration or deceleration from a speed, then the firing time becomes displaced only with increasing or decreasing speed. This results in a dragging change in speed. For this reason, the firing time is not always adapted to the aspirated mixture (lean or enriched) and this causes an unfavorable exhaust gas quality.
Particularly for two-stroke engines, it is especially the speed stability at idle which is problematical; however, it is also not easily set at high speed and at the highest speed. Although the known control of the firing time can be improved, the sought-after stability is, however, not always completely obtainable.